It is widely acknowledged that an integrated approach is required to bring about a long-lasting reduction in schistosomiasis transmission. One promising component of control is the introduction of parasite- resistant snails into endemic areas to replace the resident, susceptible snails and avoid the often destructive changes in the local ecosystem that accompany other forms of snail control. Before this type of control is attempted we must address a number of vital issues. For example, it is important to understand the nature of the snail genes responsible for conferring resistance against the parasite, to develop markers for detecting the frequency of resistance genes in field populations, and to document some of the growth and breeding characteristics of resistant snails when in mixed populations. We propose to use a number of methods in molecular analysis to identify the relevant Biomphalaria glabrata snail genes involved in resistance/susceptibility to Schistosoma mansoni, to determine their chromosomal location, and to identify developmentally regulated transcripts after parasite exposure and the tissues in which these are expressed. To complement these efforts, we hope to further define S. mansoni genes involved in snail infectivity. All of these studies are geared toward a better understanding of the molecular processes involved in influencing parasite development in the snail host, and how this information eventually can be applied in natural settings for an effective and potentially low-cost biological control strategy for human schistosomiasis.